1. Related Applications
The present disclosure is directly or indirectly related to the following U.S. and Japanese applications:
A U.S. patent application titled "Apparatus for Navigating Vehicle" (filed in the United States on Apr. 25, 1989, Ser. No. 07/342,951); and Japanese Patent Applications (on which this U.S. application is based) No. 63-106556 (filed in Japan on Apr. 28, 1988), 63-108175 (filed in Japan on Apr. 30, 1988), and 63-1087176 (filed in Japan on Apr. 30, 1988).
2. Field of the Invention
This invention relates to a navigation apparatus, and, more specifically, to a vehicle navigation apparatus of the type which helps to guide a vehicle while matching the current vehicle position conjectured by the dead-reckoning method with the road as depicted on a map stored previously.
3. Description of the Prior Art
Recently, a dead-reckoning method has come to be used in the field of vehicle navigation. According to this method, a current vehicle position is conjectured on the basis of information on the vehicle travel direction obtained by utilizing geomagnetism and the integrated travel distance. In navigating a vehicle by this dead reckoning method, the current vehicle position conjectured by the method is displayed on a display screen along with a map representing the area around the position. It is said to be a problem with this method that it can involve accumulation of measurement errors.
Various attempts have been made to overcome this problem. For example, there is a method available which is called map matching. As described in Japanese Patent Laid-Open No. 61-209316, this map matching method consists in correcting the conjectured current vehicle position to a position on a road map which exhibits a peculiar road configuration, such as an intersection, at the time when the vehicle is judged to have passed that position (The operation is referred to as map matching).
It is to be noted, however, that, after all, this map matching method is also based on the dead-reckoning method. Furthermore, peculiar points such as intersections cannot always be expected to exist. It can happen that two adjacent peculiar points are separated from each other by such a distance that measurement errors accumulate in the interval from one to the other to an excessive degree, so that map matching cannot be conducted any more due to the accumulated measurement errors. Attempting to perform map matching in such cases may result in the current position being erroneously map-matched onto a road other than the one on which the vehicle is actually running. Once the current vehicle position has been map-matched onto the wrong road, it is impossible to effect correct map matching thereafter.
In view of this, the inventors of the present invention developed a map-matching method in which map matching can be reliably conducted without depending on such unreliable points as intersections, which cannot always be expected to exist, and then filed patent applications for the method in the name of the assignee (e.g., the U.S. patent application titled "Apparatus for Navigating Vehicle", filed on Apr. 25, 1989, Ser. No. 07/342,951, and Japanese Patent Application No. 63-106556). This method comprises a combination of the dead-reckoning and map-matching methods, and further employs a plurality of nodes which are arranged along the roads on a map and used as data. Coordinate positional information is assigned to each of these nodes. Furthermore, a link relationship is established to relate each node to the others which are connected thereto through roads. Once such a link relationship has been established, it is logically impossible for a vehicle to start from one node and reach another which is not linked therewith. The peculiar points including intersections as mentioned in Japanese Patent Laid-Open No. 61-209316 may be regarded as "nodes" which are not link-related to each other.
In accordance with the disclosures of Japanese Patent Application No. 63-106556, etc., a multitude of such nodes are provided, which allows map matching to be conducted periodically, thereby making it possible to effect map matching before vehicle position measurement errors have accumulated to an excessive degree. Accordingly, there is no danger of a vehicle position being corrected to the wrong road. In the following description, roads which are previously stored in memory in terms of an inter-node link relationship will be referred to as "memory roads".
However, to enable periodical map matching using nodes provided along roads as in Japanese Patent Application No. 63-106556, it will be necessary to provide nodes for all the roads in the area concerned that are open to vehicles, including tiny back alleys. This, however, is impossible because of the limited capacity of the memory means (e.g., CD-ROM) for storing road information and the high cost involved. In view of this, it might be possible to prepare nodes for only those roads on which the vehicle is likely to be found traveling relatively often, and eliminating nodes for smaller roads. If, in that case, the vehicle travels on a road to which no nodes are allocated, it is to be regarded as "non-memory road travel" which allows no map matching. Attempting to perform map matching in such cases will result in the vehicle position being erroneously map-matched onto a memory road, which means that the vehicle position as displayed on the navigation apparatus is in disagreement with the road on which the vehicle is actually running. Japanese Patent Laid-Open No. 61-209316 takes no account of such non-memory road travel.
Japanese Patent Application No. 63-106556, mentioned above, solved this problem by developing a control method comprising a procedure for finding a target node when the vehicle concerned is considered to be traveling on a memory road, a procedure for making a judgment as to whether the vehicle is traveling on a road which is not stored in memory (hereinafter referred to as a "non-memory road"), a procedure for finding a target node when the vehicle is traveling on a non-memory road, and a procedure for making a judgment as to whether or not the vehicle has been restored to a memory road. When, as shown in FIG. 1, the vehicle is judged to be traveling on a non-memory road, the distance Lx between the point (referred to as Px) at which it is considered to have deviated from a memory road and the current position (referred to as Po) conjectured by the dead-reckoning method, is calculated, the target node being searched for on the basis of this distance. The term "target node" represents here a single node that is to be specified from among the nodes which are within a circle having the point Px as its center and a radius r that is expressed as: EQU r=Lx.times..beta..
Such nodes will be hereinafter referred to as "candidate nodes". In searching for the target node, the distance Lx' from the current position Px to each candidate node is calculated. When the distance Lx' from the current vehicle position to a candidate node is less than a certain threshold H, i.e., EQU Lx'&lt;H,
that candidate node is regarded as the node to be reached. In FIG. 1, the solid line represents a memory road, and the dashed line a non-memory road.
Thus, in accordance with this method, the target node is specified, when the vehicle is running on a non-memory road, from among candidate nodes that can be reached, on the basis of the distances Lx' between the current position and the respective candidate nodes. As will be understood, this method simply depends on the comparison of the respective distances Lx' and the threshold H, resulting in rather low accuracy in specifying the target node. Accordingly, it can happen that the wrong node is specified as the target node to be reached when the vehicle is restored to a memory road, which makes it impossible to continue dead reckoning and map mapping thereafter.